1. Field of the Invention
The present invention relates to a staged system for producing purified hydrogen, from a reaction gas mixture comprising a hydrocarbon compound, such as for example an alcohol or synthesis gas containing carbon monoxide.
2. Description of the Prior Art
Generally, it is known that purified hydrogen is used in the manufacturing of many products, such as metals, semiconductors, and micro-electronics. It is also an important source of fuel for many energy conversion systems. For example, fuel cells use hydrogen and an oxidizer for producing an electric potential. Various methods may be used for producing hydrogen. One of them is the steaming of hydrocarbon feeds.
In order to produce purified hydrogen, systems which involve a reactor comprising a catalytic area for producing hydrogen have already been proposed, wherein a hydrocarbon feed reacts with steam forming a gas mixture rich in hydrogen. Selected extraction of the hydrogen contained in this mixture is achieved by specially designed membranes for separating hydrogen from the other gas components of the mixture.
The solutions proposed up to now are of the continuous type and usually apply a device (membrane) for separating hydrogen, positioned along the gas flow in the catalytic area. Thus, the gas reacts and hydrogen is permanently extracted along the reactor, as soon as it is formed and reaches the separation device.
It is found that these solutions have the following drawbacks, even in the case when the reactor is made in several sections:                The catalyst should be placed in the intimate vicinity of the membrane, otherwise mass transfer of hydrogen towards said membrane may be limiting.        The constitutive materials of the catalyst and of the separator should therefore be compatible.        The filtration device should be sufficiently fast, i.e., having sufficient flux, so that extraction of hydrogen has a large influence on the gas composition at each location. If it is too fast, then a certain portion of its surface will only see very little flow because the partial pressure of hydrogen upstream will be kept at a low level.        If the catalyst is too fast, then equilibrium is achieved as soon as the gas enters the reactor, the following section sees the extraction of hydrogen and the change in the gas composition. The catalyst is useless there. As soon as the composition has sufficiently changed, the catalyst resumes its utility for achieving a new equilibrium corresponding to the entry parameters in the third section, and so forth.        
The two following cases are summarized in the following way:                Either the hydrogen fraction in the reactor is constantly low because of the rapidity of the membrane, but then the flux flowing through is low. The membrane is under-utilized.        Or the catalyst is fast and acts intermittently with surges, waiting for the hydrogen composition to be depleted. Consequently, large catalytic areas are useless. The catalyst is under-utilized.        
The design of such systems should be investigated in detail in order to achieve significant performances in terms of compactness and cost, and the parameters which may be acted upon, remain few (amount of catalyst per reactor unit length, size of the channels).